Accompanying the radio (or CD player, or iPod, or victrola...)

not the one I'd remembered but here's one related thread:

Good songs to play?
http://www.thereminworld.com/forum.asp?cmd=p&T=11&F=1

Fast Happy songs are good on the theremin too!

A common thread at the beginning seems to be to try out pieces you know well and can sing on your own. Those are already in your ears and your hands will follow quicker than things you're learning from scratch.

Keep something you know an play well in your daily line up, it helps keep practice fun and solidifies your technique for those new pieces that may take more sweat.

Playing a capella is as hard on the theremin as it is singing. (For some lucky folks it's not.) I often find the same bug-a-boos that haunted me there were waiting for me at the theremin.

I agree that playing with recordings or live musicisna can help a lot.

It forces you to be on pitch.

A capalla playing is important, though, too. so try to do a mixture of both.